A pro choice person wouldn't judge them for "making a mistake." Calling an unplanned pregnancy a major life mistake, is your judgement. A pro choice person would consider teen mothers as "making their choice and deciding what to do with their pregnancies." It is their choice, and should be their choice to become a parent. If a teenager chooses to raise a child, then they need to take responsibility for raising that child. Loving your own child and raising it with compassion is nothing something you can force an anybody including a teenage girl. They either have that personal goal, or they don't.
The pregnant teens I went to school with have had very different, and very difficult lives, because of their teenage pregnancies. The teenage father usually does not stay in their lives, so they need to be prepared to be a single parent. They are less likely to graduate high school and go to college. My best friend has since graduated college, but she did so, very, very late. My other friend got her GED, and is in college and doing other things.
Their lives are very different from mine. They make less money than me, and have kids to take care on top of that. However, they made the choice to parent. This was all their CHOICE. I don't consider their children mistakes. I don't consider their reproductive decisions mistakes. Teenage pregnancies are very tragic. However, the only way I would support a child taking on such a responsibility, is if they choose it for themselves and they are willing to be a good parent. I see no point in judging them and pointing out mistakes. I always supported them, and I always will.